docs: def Steiner(x, rhol, rhog, sigma, m, D, g=g):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_ also given in [2]_ and [3]_.
      
      .. math::
          \alpha = \frac{x}{\rho_g}\left[C_0\left(\frac{x}{\rho_g} + \frac{1-x}
          {\rho_l}\right) +\frac{v_{gm}}{G} \right]^{-1}
          
      .. math::
          v_{gm} = \frac{1.18(1-x)}{\rho_l^{0.5}}[g\sigma(\rho_l-\rho_g)]^{0.25}
  
      .. math::
          C_0 = 1 + 0.12(1-x)
          
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      sigma : float
          Surface tension of liquid [N/m]
      m : float
          Mass flow rate of both phases, [kg/s]
      D : float
          Diameter of the channel, [m]
      g : float, optional
          Acceleration due to gravity, [m/s^2]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ has been reviewed. 
          
      Examples
      --------
      >>> Steiner(0.4, 800., 2.5, sigma=0.02, m=1, D=0.3)
      0.895950181381335
  
      References
      ----------
      .. [1] Gesellschaft, V. D. I., ed. VDI Heat Atlas. 2nd edition.
         Berlin; New York:: Springer, 2010.
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      G = m/(pi/4*D**2)
      C0 = 1 + 0.12*(1-x)
      vgm = 1.18*(1-x)/rhol**0.5*(g*sigma*(rhol-rhog))**0.25
      return x/rhog*(C0*(x/rhog + (1-x)/rhol) + vgm/G)**-1